The present invention relates in general to solar energy systems in which solar energy is directly absorbed by a working fluid, and in particular to systems with heat engines.
Solar energy systems associated with heat engines may be based either on non-direct or on direct absorption of concentrated solar radiation energy by a working fluid.
In solar energy systems with non-direct absorption, the energy of concentrated solar radiation is absorbed by ceramic or metallic absorber bodies in an absorbing chamber where working fluid is heated by the absorber bodies and circulates between the absorbing chamber and a heat engine (U.S. Pat. Nos. 5,849,838, 4,320,743). In such systems there exists a substantial temperature difference between the solar absorber bodies and the working fluid, which may cause boiling of the working fluid at the area of its contact with the absorber bodies, reducing the local heat transfer coefficient and causing hot spots. When the working fluid is in the form of an organic liquid, as preferred for various heat engines, decomposition or carbonization of the liquid may occur at the areas of its contact with the absorber bodies that may completely paralyze the system. For this reason, solar energy systems with organic working fluids have a maximal allowed working temperature of approximately 400xc2x0 C. However, this limit may not be closely approached due to variations of the solar radiation intensity, which does not allow the use of highly concentrated solar radiation.
In solar energy systems that are based on direct absorption of the energy of concentrated solar radiation by the working fluid (U.S. Pat. Nos. 4,055,948, 5,214,921, 4,286,581), the working fluid is directly heated to high temperatures at a high pressure absorption chamber and is subsequently transferred to a boiling chamber where it expands and evaporates, for the vapor to be used in a heat engine. In some of the direct absorption systems, the working fluid is in the form of a suspension with solar radiation absorbing particles melting into liquid phase upon being heated by concentrated solar radiation.
The present invention suggests a direct absorption device in which solar radiation energy is absorbed directly by a working liquid and is transformed into the latent heat of boiling the liquid to form vapor whose pressure is controlled to make the working liquid boil at a predetermined temperature.
In accordance with the present invention, there is provided a solar energy system comprising a solar absorber in the form of a solar boiler tank with a lower working liquid region having a liquid inlet and filled with a working liquid capable of absorbing highly concentrated solar radiation and boiling thereupon under a predetermined pressure, and an upper vapor accumulation region having a vapor outlet for withdrawing from the tank a vapor created in the tank, and vapor utilization means associated with said vapor outlet, said solar boiler tank having at least one transparent window to receive and pass towards the working liquid highly concentrated solar radiation, the system further comprising means for controlling the pressure of vapor in said vapor accumulation region to make the working liquid boil at said predetermined pressure.
Since the absorbing factor of working liquids is normally low, their absorption of concentrated solar radiation inside the solar boiler tank, according to the present invention, may be easily controlled avoiding the risk of overheating and disintegration of the working liquid.
Preferably, the working liquid is an organic liquid, which enables the use of highly concentrated solar radiation (up to 1000 and even more) and, consequently, small dimensions of the absorber. In this case, the vapor utilization means may comprise a heat engine where the organic vapor is used to produce mechanical power and optionally electrical power and, when condensed there, is introduced by means of a high pressure feed pump, back into the solar boiler tank through the working liquid inlet. When the vapor utilization means is of the kind where a working fluid other than the organic liquid is preferred, which however at high temperatures develops high working pressures, the organic liquid may still be used as a working liquid in a primary cycle of production of the organic vapor whilst in a secondary cycle, the vapor""s heat will be utilized in any appropriate manner for heating or evaporating said other working fluid. Such a design may be suitable for use with the secondary cycle working fluid being water steam.
Preferably, the solar energy system of the present invention comprises a solar radiation concentration system in the form of a field of heliostats and an additional concentrator such as a CPC associated with said at least one window. In this case, it is preferable that the solar boiler tank has a plurality of windows each facing a group of heliostats or one heliostat and each provided with said additional concentrator. This design enables the use of small windows and provides for any required concentration within the thermodynamic limit.
The windows in the solar boiler tank of the present invention may be made of any material suitable for working temperatures and pressures of the system without any concern of overheating because the windows are immersed in the working liquid.
The solar boiler tank may be disposed above the field of heliostats (on a solar tower or on a hill), in which case said mirrors are formed at the lower region of the solar boiler tank. The vapor utilization means in this case does not need to be located in the vicinity of the solar boiler tank but rather may be remote therefrom, e.g. on the ground level, with the vapor being delivered thereto via a pipe. The solar boiler tank may also be disposed at the ground level, in which case the window(s) should preferably be formed at the upper region of the solar boiler tank. In fact, in the latter case, the window(s) may be formed at any location of the solar boiler tank, provided the solar radiation concentration optics is built to thereto direct concentrated solar radiation enabling all this radiation to pass towards the working fluid.
The pressure control means in the system according to the present invention may be in the form of a pressure controller adapted to regulate the pressure of the vapor in the solar boiler tank or in the form of a heat controller adapted to regulate the quantity of said concentrated solar radiation incident on said window. The latter may be obtained by regulating the number of operative heliostats at different times of the day.
The system according to the present invention may be provided with heat storage means in the form of salts or metal alloys located at the bottom of the lower region of the solar boiler tank, that are compatible with the working liquid and capable of changing their phase when heated to temperatures at which the working fluid is designed to boil.